It is often desired to dry biological samples in order to preserve their shelf life and activity. One drying technique is lyophilization, which is a commonly employed freeze-drying technique. Still other biological materials, such as long chain DNA molecules and cell components, are desired to be dried at a temperature above 0.degree. C., i.e., above freezing, in order to prevent their destruction by the forces of freezing. Inasmuch as the present invention is not limited to lyophilization, drying above and below the freezing point are discussed interchangeably.
Usually, after a lyophilization process is completed the freeze-dried compound is stored in a freezer, e.g., at -70.degree. C., although lyophilization can sometimes obviate the need for freezing all together. For example, according to the manufacturer's product profile sheet, Endothelial Cell Growth Supplement (ECGS) is stable for at least 18 months when stored at 4.degree. C. in lyophilized form, but only one month when stored in a solubilized form at -20.degree. C.
Lyophilization of compounds is particularly useful when growing cells in a culture medium where the lyophilized compounds include peptides or growth factors. These compounds are generally provided in minute quantities due to their expense and/or potency, and they are usually extremely perishable. Lyophilization is observed to extend their shelf life.
Typically, lyophilization is carried out in a centrifugal apparatus, such as a Speed-Vac.RTM. centrifuge. The Speed-Vac.RTM. is placed in a vacuum chamber. The sample is placed in a microcentrifuge tube which is a small plastic tube (0.5, 1, or 2 mLs) typically tapered, conical or rounded, and closed at one end. Because the vacuum used in lyophilizing is extremely high (e.g., 50-500 milli.Torr), some of the liquid in the microcentrifuge tube vaporizes immediately and forces out much of the remaining solution from the tube. By applying a centrifugal force, the liquid is pushed down to the bottom of the tube in an effort to prevent the liquid from jetting out when the liquid gassifies. After lyophilization is complete, the vacuum is turned off, thereby allowing the vacuum chamber and the interior volume of the tube to return to ambient pressure.
In order to use stored dried compounds, they must be dissolved (if not already stored in solution), then filtered-sterilized, which filters out all living cells, dust, and other unwanted materials. The volume of the solution at this stage is small, e.g., 1 ml. After filter-sterilization, the compounds are usually distributed in aliquots, e.g., of 50 .mu.l each, and unused aliquots are stored in a freezer. This avoids the necessity of repeatedly freezing and thawing the compounds, which shortens their shelf life.
Another lyophilization method entails leaving the microcentrifuge tube lid open during lyophilization. After the vacuum is terminated, the lid is then closed. This method produces an unsterile sample, which must be resterilized by filter-sterilization. However, in this method, that portion of the stored sample adsorbed to the filter is lost.
Still another method is to perform lyophilization in a sterile environment such as a clean room. However, this requires incurring the additional expense of maintaining a clean room environment.
Yet another method proposes sterile gas exchange through a membrane in an enclosed sterile environment, see, for example, U.S. Pat. No. 5,398,837 and a cell culture flask manufactured by Costar (catalog number 3056). However, neither of these methods is suitable for lyophilization using a centrifuge since the cell culture flasks cannot be centrifuged at high speeds. Moreover, the cell culture flasks provide a slow gas exchange between the outside environment and the cell culture being grown. Furthermore, the porosity of the membrane is such that it is permeable to gas but not to microbes, e.g., having diameters above about 0.22.mu..
Accordingly, a need exists for a container for a material that can be subjected to high centrifugal forces, as during a drying procedure, but which permits sterile gas exchange between the interior of the container and the external environment. Such a container need only be capable of permitting drying while preventing microbial contamination, independent of centrifugation, for those applications not requiring centrifugation.